I can't speak about the new hybrids that have been produced, but the old varieties that have been in the hobby for the last 30 or more years do quite well with low to moderate light (one to three watts of ordinary T-12 fluorescent light per gallon) and no additional CO2. They are not particularly fussy about high nutrient levels. if you keep the potassium level higher than zero and calcium & magnesium higher than about 20 ppm, even a low fish load provides the rest of the nutrients. They seem to be very good at getting iron. About all they need is a small amount of soil in the gravel. Any kind of commercial substrate that has iron already is more than adequate. MY experience has been with E. amazonicus, E. bleheri, E. bolivianus, E. martii, E. osiris, E. parviflorus and E. uruguayensis. All on my list except E. bolivianus are big and tend to take over a smaller tank entirely and a significant part of a larger tank. Oher plants that can live with them have to be tough, also.
One of the most impressive examples I have ever seen of getting nutrients where none should be available was an E. bleheri (or amazonicus---I can't tell the difference) I had when I was in high school and just starting aquariums in the 1950's. The plant was in a 29 gallon, planted in washed quartz aquarium gravel. The water was very hard, and there was only one zebra fish in the tank! That fish was the only source of nutrients, and yet the plant grew well and filled the tank. I don't know where it got its iron from!
While on the subject of swords, would running CO2 be a must in a 180g with 2.5 watts of MH, if they were the only type of plant in the tank? Just curious?These are large plants in 5" pots before planting. Would they still look good without the CO2 ? I know they are tough, but are they that tough?
I have had very little experience with swords and CO2. Probably the CO2 would speed up their growth and make them demand somewhat (not very much) higher nutrient levels. They look pretty good with just the CO2 levels provided by a low to moderate fish load.
i have grown my fair share of swords, and IMHO, if you add co2 you may want to consider more nutrients. swords are hogs when it comes to nutrients. that is why many people say to take them out of stem tanks b/c of the uptake of nutrients. and adding co2 will just require them to need more. otherwise, i have noticed that the leaves start to suffer. but they do great without the co2 too!
My small bleheri sword did not like growing in my 1.6 wpg low light 25 gallon tank at all. It produced some new leaves but stayed a very short height, like only about 4 inches. The mother plant this baby came off, in moderate light has gotten massive.
In conclusion, I've found that echinodorous bleheri is a fairly light demanding plant.